1. Field of the Invention
This invention relates to a global positioning system (GPS) receiver which receives multiple ranging signals to calculate both the position and the attitude of a vehicle. More specifically, this invention relates to the structure of a position processing apparatus which uses the direct code sequence signals from GPS satellites to determine the position and the attitude of a moving vehicle.
2. Discussion of the Related Art
GPS receivers are widely used as navigational tools. An advanced GPS receiver installed in a vehicle, such as an airplane or a boat, receives code sequences from satellites which allows the GPS receiver to measure not only the position but also the attitude of the vehicle. Attitude is defined as the orientation of a vehicle as determined by the relationship between its axes and some reference axes. For vehicles that travel on the surface of the Earth, the reference axes are normally the meridians and the parallels of the Earth. In an aircraft, on the other hand, a third reference axis, an altitude line perpendicular to the surface of the Earth is also used. Thus the attitude of an aircraft includes three components: roll, pitch and yaw.
Compared to a position calculation, an attitude calculation is very sensitive to errors. For example, the antennae receiving the different satellite code sequences must be placed apart from each other at a distance less than the carrier wavelengths of the GPS signals, absent the use of some sophisticated anti-ambiguity technique. Thus, since such carrier signals are typically no more than a few inches long, a small, non-systematic error in a distance measurement may result in a large error in the attitude calculated.
One conventional technique for calculating attitude uses a fully redundant carrier tracking circuit. Under this approach, a separate code sequence processor is provided for each of the antennae, so that each code sequence is assigned to be processed concurrently and independently of the other code sequence processors. Because of this fully redundant design, a GPS receiver built under this approach is typically large and expensive.
One method to reduce the size and cost of such a GPS receiver is taught in U.S. Pat. No. 5,268,695 ("Dentinger '695 patent") to M. Dentinger, filed Oct. 6, 1992, issued on Dec. 7, 1993. The Dentinger '693 patent teaches multiplexing the antenna signals through one processing unit, where code sequences from different antennae are acquired at different times and are compared against each other. However, since the attitude of an aircraft changes between code sequence acquisitions, the Dentinger '695 patent's approach is unsuitable for use in an aircraft or a similarly fast-moving vehicle. Moreover, since each code sequence is processed under the control of a different clock signal, the Dentinger '695 patent's approach is not capable of concurrent common-phase error cancellation, thereby resulting in a significant error in the calculated attitude.
To synchronize with the received code sequence of a GPS signal, a GPS receiver typically uses a phase-locked loop to obtain a "code lock" using a locally generated code sequence. Code lock can be achieved by minimizing the phase "lead" of the local code sequence relative to the received code sequence ("early lock"), or by minimizing the phase "lag" of the local code sequence relative to the received code sequence ("late lock"). For a GPS receiver with a wide correlation width, i.e. a GPS receiver capable of achieving code lock over a wide phase range, the lock condition is more susceptible to signal distortion such as multipath which can result in an undesirable error in the pseudo range calculation. Thus, some GPS receivers use a narrow correlation width. However, when noise or a phase drift is present in the GPS carrier signal, the narrow correlation width used in this technique is often inadequate to provide a stable code lock, resulting in frequent out-of-lock conditions.